Maybe some things, if it may help :
G’MIC uses calls to the library libcurl to retrieve files from the G’MIC domain https://gmic.eu , as a few filters require external data to be loaded (loaded files go to the G’MIC resources folder, which is often $HOME/.config/gmic ).

Thanks @David_Tschumperle. After some more investigations, it turn out that the error returned by libcurl is CURLE_SSL_CACERT_BADFILE. That is, the CA certificate is not accessible for some reasons… I have to see if that is a problem of environment variables.

@ggc@s7habo@Bilbo@David_Tschumperle
After more investigations, it turn out that there is a mismatch between the LibCURL version that is bundled in the AppImage (and used by G’MIC to download the CLUTs) and the CA certificates that are available in systems like Ubuntu 18.04

The more reliable solution I have found is to disable the LibCURL support in G’MIC/CImg, and let the code fall back to external CURL/WGET commands for downloading network files. Either curl or wget should normally be present on all Linux systems, so that should not be a big limitation.